1. Field of the Invention
The present invention relates to an improved method and apparatus for recovering and simultaneously partially refining hydrocarbon products, such as liquid oil, oil vapor and combustible and noncombustible gases, from oil shale. Valuable by-products such as sulfur and rare earth minerals and elements, are also recoverable by this method. More specifically, the invention relates to a method and apparatus for obtaining hydrocarbon products or fractions from oil shale, by the use of microwave energy.
2. Description of the Prior Art
Oil shale is commonly defined as organic-rich shale that can yield oil or gas upon heating. (T. F. Yen, "Science and Technology of Oil Shale", Ann Arbor Science Publishers, Ann Arbor, Mich.) Reference has been made in the art and published literature to the recovery of products from coal and oil shale by the use of microwave energy. It has been observed that certain organic solids, notably hydrocarbon fuel precursors such as coal and some similar substances, such as oil shale, can be destructively distilled by the application of microwave energy at high energy levels. R. F. Cane, NATURE, Jan. 8, 1966, pages 197, 198. As described by Cane, the organic matter was reduced to coke by the action of microwave energy. The microwave distillation and cracking of crude oil and shale oil, and the decomposition of coal and oil shales is mentioned in the ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, 1972 Supplement, at page 576. This latter reference also refers to Belgian Pat. No. 481,314 (1948). Studies on the pyrolysis of oil shale by microwave radiation heating have, subsequently, been reported by W. D. Stuckey, "A Study of the Pyrolysis of Oil Shale by Microwave Heating", M. S. Thesis, University of Colorado, Boulder, Colo., 1977. The results of further investigations and evaluations were reported in a technical paper by Chia-Lun J. Hu, "Online Measurements of the Fast Changing Dielectric Constant in Oil Shale due to High Power Microwave Heating", IEEE Transactions on Microwave Theory and Techniques, Vol. MIT-27, No. 1, Jan. 1979. The studies and investigations of Stuckey and Hu were supported in part by a National Science Foundation Grant (NSF Program-RANN No. AER 75-17453 awarded from May 15, 1976 to May 31, 1978 and subsequently extended to Aug. 31, 1979 by the United States Department of Energy).
Microwave power or energy refers to the wavelength or frequencies occupied in the electromagnetic spectrum. The domain occupied by the microwave frequencies in the electromagnetic spectrum is of arbitrary extent, between approximately 10.sup.8 and 10.sup.12 Hertz (cycles per second). This frequency interval thus adjoins the infrared frequency, which is usually considered to have a low-frequency limit of 10.sup.12 Hertz. The upper frequency limit of radio communication is of the order of 10.sup.9 Hertz. The propagation of wave energy in free space is of the order of 3.times.10.sup.8 meters per second. Presently, typical and representative microwave power can be generated which has a frequency of between 0.915.times.10.sup.9 and 2.45.times.10.sup.9 Hertz. The wave lengths associated with these frequencies are, respectively, 0.328 meters and 0.122 meters, which are two of the four bands designated by the Federal Communications Commission (FCC) as the industrial, scientific and medical (ISM) frequency bands. If the FCC approves, lower frequencies such as those doen to 0.300.times.10.sup.9 Hertz are included.
In a microwave heating system, the electrical energy input is converted to direct current and then applied to a high power microwave tube which generates the microwave power. This high frequency energy is transmitted to an applicator through a wave guide. The applicator is a wave guide element designed to efficiently couple or direct the microwave power or radiant energy into the material to be heated.
Process transducers are conventionally used to measure the microwave field, process rate, and operating temperature. The operating microwave power level is controlled by the control unit thus permitting a constant process through closed loop feedback control. Unlike other forms of heating, microwave energy responds instantaneously to control. This follows since the heat is developed at the molecular level within the material.
The use of microwave energy for producing or inducing of chemical reactions, is well-known. In spite of the extensive research and many studies which have been made, the matter and mechanism by which microwave energy produces a reaction or atomic and molecular change within various materials is not well understood. Heating at the microwave frequency, for example, is used extensively for both domestic and commercial purposes. Microwave energy has been applied in the petroleum industry for fracturing underground geological formations surrounding low yield or depleted oil wells to reduce the resistance to flow of the liquid petroleum through the material surrounding the well bore.